This invention relates to an improved, environmentally preferred, process for making chromic acid. In particular, it relates to an improvement wherein chrome contaminated sodium sulfate or sodium bisulfate produced in a process for making sodium bichromate or chromic acid is reacted with hydrochloric acid to produce sulfuric acid, and the sulfuric acid is recycled back to the process.
Chromic acid and sodium bichromate or sodium chromate are made by roasting chromium-containing ore with soda ash (sodium carbonate) or sodium hydroxide to form sodium chromate. The roasted ore is quenched in water and the soluble sodium chromate dissolves in the water to form "yellow liquor." The sodium chromate is reacted with sulfuric acid or sodium bisulfate to produce sodium bichromate (the "bichromate process"). Upon evaporation, by-product sodium sulfate crystallizes and is filtered off. Because it is contaminated with about 200 to about 1600 ppm of hexavalent chromium, it can be sold only to low value markets such as the pulp and paper industry, and the chromium values in it are lost. Increasingly, environmental considerations limit the sale of sodium sulfate to a maximum chromium contamination of 1000 ppm.
A portion of the sodium bichromate is reacted with sulfuric acid to produce chromic acid, which precipitates and is filtered off for further processing (the "chromic acid process"). The filtrate contains sodium bisulfate, dissolved chromic acid, and excess sulfuric acid. To make use of the acid value of this stream, the sodium bisulfate filtrate is recycled to the sodium bichromate process where it is reacted with sodium chromate. The sodium bisulfate contains about 5 to about 10 wt % chromic acid which is then reconverted into sodium bichromate, a product which is less valuable than chromic acid.
The sodium bichromate produced can be used to make products for leather tanning, chrome plating, and the manufacture of pigments, as well as other uses. Chromic acid crystal or flake is primarily used in the manufacture of wood treating compounds to make the wood rot resistant.
In the chlor-alkali industry, sodium chloride brine is electrolyzed to produce caustic (sodium hydroxide), chlorine, and hydrogen. Projections indicate that in the future there will be an excess of chlorine relative to sodium hydroxide. It is difficult to dispose of or make use of the chlorine that is produced in making caustic without creating an aqueous waste stream which contains large amounts of chloride ions. To reduce the chlorine produced, the production of caustic must either be reduced or other, more expensive, ways of producing caustic must be used that avoid the production of co-product chlorine. While the chlorine can be burned in hydrogen to produce hydrochloric acid, there is also an excess of hydrochloric acid, with future projections indicating that severe excesses of this commodity are also likely to occur.